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Astrophysics

Title:Dark Matter and Big Bang Nucleosynthesis

Abstract: The recently observed Deuterium abundance in a low- metallicity high-redshift
hydrogen cloud, which is about ten times larger than that observed in the near
interstellar medium, is that expected from the Standard Big Bang
Nucleosynthesis theory and the observed abundances of $^4$He and $^7$Li
extrapolated to their primordial values. The inferred cosmic baryon to photon
ratio, $η=(1.60\pm 0.1)\times 10^{-10},$ yields a mean cosmic baryon
density, in critical mass units, of $Ω_b\approx (0.6\pm 0.1)\times
10^{-2}h^{-2},$ with $h$ being the Hubble constant in units of $100 km~s^{-1}
Mpc^{-1}$. This baryon density is consistent with the mean cosmic density of
matter visible optically and in X-rays. It implies that most of the baryons in
the Universe are visible and are not dark. Combined with the observed ratio of
baryons to light in X-ray emitting clusters, it yields the value $Ω
\approx 0.15$ for the mean mass density of the Universe, which is consistent
with that obtained from the mass to light ratio in clusters. This mass density
is about ten times larger than the mean baryon mass density. It indicates that
most of the matter in the Universe consists of nonbaryonic dark matter.

Comments:

Invited talk to be published in the proceedings of ``Dark Matter In Cosmology'' (Villars sur Ollon, Switzerland January 21-28, 1995)